A hearing aid is a system used by a hearing-impaired person, a person with failing hearing, and the like to compensate for hearing. The hearing aid converts an external acoustic signal to an electric signal by a microphone, amplifies a level of the electric signal, converts the amplified electric signal to an acoustic signal again by a receiver like an earphone, and outputs the acoustic signal as audible sound that can be heard by the user.
The acoustic signal acquired by the microphone includes not only sound information necessary for the user such as conversational speech, television or radio output sound, and an intercom or telephone ring, but also various undesired sound, such as daily life noise and environmental noise, that interferes with recognition of the sound information necessary for the user. In view of this, various techniques of combining amplification and attenuation to ease the user's hearing have been devised for the hearing aid, including nonlinear amplification processing of amplifying low-level sound and not amplifying high-level sound.
In particular, a digital hearing aid that converts an acoustic signal acquired by a microphone to a digital signal and performs hearing aid processing by digital signal processing is provided in recent years. For example, there is provided a hearing aid that performs advanced noise suppression processing by dividing a acquired signal into a plurality of bands, discriminating between a desired signal and an undesired signal (for example, speech and non-speech) for each band at high speed, and extracting only the desired signal (for example, a speech signal). There is also provided a hearing aid that has a function such as directional sound acquisition of extracting only an acoustic signal coming from the front by using an input time difference between microphones placed at two positions in front and back of the hearing aid. There is further provided a hearing aid that has an internal storage area storing a plurality of hearing aid algorithms, and switches between a plurality of hearing aid processing automatically or manually by the user according to a surrounding environment of the user.
There are conventionally a number of proposals for the concept of switching between a plurality of hearing aid processing according to the surrounding environment of the user. For instance, a hearing aid having a structure shown in FIG. 1 analyzes the surrounding environment by applying a HMM (Hidden Markov Model) to the input acoustic signal to thereby identify/classify the surrounding environment as a predefined scene, and switches to a hearing aid algorithm corresponding to the predefined scene (for example, see Patent Literature 1). Moreover, a hearing aid having a structure shown in FIG. 2 analyzes constancy of ambient noise, and either switches between directional processing and noise suppression processing that employs spectral subtraction or simultaneously activates both processing, thereby improving speech clarity according to ambient noise quality (for example, see Patent Literature 2).
A conventional hearing aid 1001 shown in FIG. 1 is a type of hearing aid that performs hearing aid processing in a hearing aid processing unit 1003 for an acoustic signal acquired by a microphone 1002, and outputs the processed acoustic signal from a receiver 1004. In the hearing aid 1001, a signal analysis unit 1005 extracts acoustic features from the acoustic signal, and a signal identification unit 1006 identifies an instantaneous acoustic environmental situation. The hearing aid processing unit 1003 switches between a plurality of hearing aid algorithms according to the acoustic environmental situation identified by the signal identification unit 1006. The identification of the instantaneous acoustic environmental situation by the signal identification unit 1006 is conducted on the basis of a combination of hearing-based features such as a sound intensity, a spectral pattern, and a harmonic structure extracted by the signal analysis unit 1005, with the HMM being employed as an identification algorithm. The HMM is a statistical approach widely used in speech recognition and the like, and is a probabilistic model that estimates an output state for an unknown input, from an occurrence probability distribution in each state and previous state transitions. To apply the HMM, a training device 1007 for appropriately initializing a parameter so as not to fall into a local optimum is needed.
A conventional hearing aid 2001 shown in FIG. 2 is a type of hearing aid that performs hearing aid processing on an acoustic signal acquired by a plurality of microphones 2002a and 2002b in a hearing aid processing unit 2003, and outputs the processed acoustic signal from a receiver 2004. In the hearing aid 2001, a signal analysis unit 2005 calculates a signal level and constancy of the input acoustic signal acquired by the microphones 2002a and 2002b. The hearing aid processing unit 2003 either switches between directional processing and noise suppression processing that employs spectral subtraction or simultaneously activates both processing, according to the constancy of the input acoustic signal calculated by the signal analysis unit 2005. The hearing aid processing unit 2003 also switches between input-output characteristics tables of nonlinear processing, according to the signal level of the input acoustic signal calculated by the signal analysis unit 2005. This makes it possible to perform hearing aid processing only on a speech component after removing a noise component included in the input acoustic signal. Spectral subtraction mentioned here is a technique of subtracting an estimated noise component from an input signal in a frequency domain, and is a noise suppression method with an excellent capability of removing constant noise such as fan noise and background noise.